The present invention relates to a Schottky barrier diode and a manufacturing method thereof, and more specifically, to improvement of a withstanding voltage characteristic.
A Schottky junction, formed by a contact between a semiconductor (substrate) and metal (layer) having a different work function, is well known as a Schottky barrier diode, since it has a rectifying action by a barrier. This Schottky barrier diode has a low forward voltage drop and a good high-speed response characteristic, and thus is widely used as a switching power source. Furthermore, in such switching power source, the forward drop voltage of the Schottky barrier diode performing a rectification is a major factor to determine an effectiveness of the power source, and the forward voltage drop is desired to be lower, if possible. In addition, from the viewpoint of a circuit design, a withstanding voltage exceeding a rating voltage is often needed for a Schottky barrier diode.
For example, as shown in FIG. 4, a Schottky barrier diode formed from an n− type semiconductor layer 32 formed on an n+ type semiconductor substrate 31, and a Schottky metal layer 36 which forms a Schottky junction at an interface with the − type semiconductor layer 32 is considered. This metal layer is made of, for instance, Ti. An Al layer, which is an anode electrode 37, is provided in order to cover the entire surface of the metal layer. In addition, a guard ring 34, into which p+ type impurity is diffused for coming by a withstanding voltage, is provided in an outer periphery of the semiconductor substrate, and a part thereof is contact to the Schottky metal layer 36.
However, a height of the Schottky barrier in the Schottky junction, i.e., a work function difference thereof (hereinafter, referred to as “ΦBn”) is a factor to determine a characteristic of the Schottky barrier diode. The ΦBn is a unique value of each metal.
When a positive voltage is applied to the metal layer side of the Schottky barrier diode a current flows. The voltage at this time is a forward voltage VF. On the other hand, when a positive voltage is applied to the n type silicon side and a negative voltage is applied to the metal layer side, the voltage at this time is a backward voltage. Concerning a certain Schottky barrier diode to which a backward voltage is applied, there is no current flow. At this time, as the ΦBn becomes larger, the forward voltage VF of the Schottky barrier diode becomes higher, but, in contrast, a leakage current IR becomes smaller. In other words, the forward voltage VF and the leakage current IR have a relationship of a trade-off.
Therefore, as shown in FIG. 5, a structure is widely used where a plurality of p+ type regions 107 are provided in a n− type semiconductor layer 2. This enlarges a depletion layer by a pn junction upon application of a backward voltage, and thereby restricting leakage to a cathode side even if a leakage current is generated in a Schottky junction region.
For example, when withstanding voltage is 40V, the n− type semiconductor layer 2 is in need of a resistivity of 1 Ω·cm, and, when a withstanding voltage is 600V, the n− type semiconductor 2 is in need of one of 30 Ω·cm. A depth of the p+ type region 107 depends on a withstanding voltage, but, in any cases, is about 1 μm (for example, refer to Japanese Patent Application Publication 2000-261004).
As described above, in the Schottky barrier diode in FIG. 4, there is a relationship of a trade-off that, as the ΦBn is higher, the VF is higher but the IR is smaller. In addition, when the ΦBn is identical, the values of the VF and the IR vary depending on an area of the Schottky junction.
Consequently, by decreasing a resistivity ρ of the n− type semiconductor layer 32, resistances of a current path can be reduced to design a low VF.
However, in such method, a resistivity of the n− type semiconductor layer 32 underlying the p+ type region 34 to determine a withstanding voltage decreases as well. Thus, there is a problem in that it is impossible to come by a predetermined withstanding voltage since an extension of a depletion layer becomes insufficient.
In addition, in a structure shown in FIG. 5, a depth of a p+ type region 107 is about 1 μm and very shallow compared with that of an n− type semiconductor layer 2. At the same time, a concentration of the impurity of the n− type semiconductor layer 2 is set to be low in order to come by a predetermined withstanding voltage. Therefore, if a current path is narrow due to provision of the p+ type region 107, a low VF cannot be obtained.
As above, in the Schottky barrier diode, a desired characteristic can be obtained by properly selecting an area of the Schottky junction, a Schottky metal layer, a resistivity of the semiconductor layer and so on. However, it is actually difficult to obtain predefined characteristics of the VF and the IR, and at the same time come by a predetermined withstanding voltage. Because of above, a design of a Schottky barrier diode is practically made by a little sacrifice of any one thereof.
Therefore, a structure having a plurality of p type semiconductor region (junction barrier) with a pillar shape in an n− type semiconductor layer is proposed. The p type pillars are designed so as to reach an n+ type semiconductor substrate and arranged at a predetermined interval. In this structure, a depletion layer diffuses from the p type semiconductor region to a horizontal direction upon application of a backward voltage (Japanese Patent Application Publication 2005-243716). In such structure, since the depletion layer diffuses up to an inside of the p type semiconductor region, the n− type semiconductor layer 2 becomes an almost depleted region. The depletion layer diffuses almost uniformly along a depth direction (a vertical direction to the substrate) of the p type semiconductor region to be pinched off for maintaining a constant intensity of an electric field. Therefore, an electric field applied to a Schottky junction interface can be mitigated to restrict a backward leakage current.
In the Schottky barrier diode with such junction barrier, as shown in FIGS. 5 and 6, a structure has been proposed in which a width W between the junction barriers is set to satisfy 2w0<W<3D (where, w0: a width of the depletion layer, D: a depth of the junction barrier), and thus Schottky characteristics (a forward voltage VF, a backward leakage current IR) can be improved by a pinch-off effect (Japan Patent Application publication H7-50791).